Buried heterostructures (BHs) are used in a number of semiconductor devices, for example distributed feedback and Fabry-Perot lasers. A buried heterostructure is formed by etching a semiconductor wafer to form a mesa and then depositing overgrowth layers of semiconductor material around the sides of the mesa. Suitable selection of the materials and the doping of the overgrowth layers provide the desired performance of the device. It has been observed that the dopants from the overgrowth layers have a tendency to become deactivated from the overgrowth layers and then to diffuse out of the overgrowth layers. If some of these dopant atoms diffuse into the active region of the semiconductor device then the performance and reliability of the semiconductor device may be affected and the yield of the fabrication process will be reduced.
There have been a number of potential solutions to these problems, which attempt to prevent or reduce the diffusion of dopants and in particular the diffusion of zinc. These solutions have attempted to reduce the dopant concentration within the overgrowth layers or implementing multiple layered overgrowth layers comprising undoped layers or layers having graded dopant concentrations. It has been found that such solutions tend to have a limited effect in reducing the diffusion of dopants and can lead to poor device performance, such as increased device resistance and poor performance at high temperatures.
Our co-pending patent application EP 02253129.7 describes a technique in which an oxide layer is deposited on the sides of the mesa of a buried heterostructure. The oxide layer reduces dopant diffusion into the active layer.